1. Field of the Invention
The present invention relates to a method for inspecting a state of an optical system of an exposure apparatus for use in a semiconductor photolithographic process, an exposure method for correcting a focal point, and a method for manufacturing a semiconductor device by using an exposure apparatus.
2. Description of the Related Art
In a general photolithographic process, when forming a fine resist-pattern by using a projection exposure apparatus (stepper), unless a state of an optical system of the exposure apparatus, especially a focal point of the exposure apparatus is set in an appropriate state, it is likely to be out of focus, and it is hard to form a fine pattern as desired. Recently, as the transfer-pattern is becoming much finer, it is very important to set the precision of the focal point of the exposure apparatus.
For example, in a semiconductor device of design rule of 0.13 μm, the focal depth is less than 0.5 μm. In this case, it is preferred to set the precision of the focal point at higher precision than 1/10 of the focal depth. Therefore, the focal point must be set at a precision of at least 0.05 μm. Needless to say, if the setting is high in repeatability, it is meaningless unless the true focal point can be measured precisely. Thus, when manufacturing a semiconductor device of which design rule is 0.13 μm, it is important to measure or monitor the focal point of the exposure apparatus at least at a precision of 0.05 μm.
As briefly described above by referring to a specific example, various techniques are developed for monitoring accurately the focal point of the exposure apparatus, for example, from a transfer-pattern by exposure.
One of such techniques is a monitoring technique by using a phase-shift-pattern. A representative example is disclosed by Timothy Brunner et al. of International Business Machine Corporation (IBM) in page 541 to 549 of Proc. SPIE vol. 2197 (1994) and in page 236 to 243 of Proc. SPIE vol. 2726 (1996).
This method uses an original plate mask 401 having a sectional structure as shown in FIG. 32. The original plate mask 401 comprises a light permeable mask main body 402, and a chromium-made shield 403, and a mask-pattern for monitoring (not shown) to be transferred on a semiconductor substrate by exposure is formed on a principal plane of the mask main body 402. As its sectional structure is shown in FIG. 32, the mask main body 402 comprises a reference plane 402a, and a plane 402b shifted in phase by 90 degrees (phase shifter plane), and the shield 403 is disposed in a boundary region of the reference plane 402a and the phase shifter plane 402b. A reference-pattern (not shown) is further disposed on the reference plane 402a. 
Using the original plate mask 401 thus explained briefly, a mask-pattern is exposed on a semiconductor substrate. At this time, if the position of the semiconductor substrate, that is, the focal point of the exposure apparatus (not shown) is deviated from the best focal point, relative positions of the mask-pattern (shield) 403 formed in the boundary region of the reference plane 402a and the phase shifter plane 402b and the reference-pattern (not shown) on the reference plane 402a being transferred on the semiconductor substrate are changed. In this case, a deviation amount of the semiconductor substrate from the best focal point and a relative position deviation amount are known to have a mutually linear relation. This method proposed by Timothy Brunner et al. is intended to monitor accurately the focal point of the exposure apparatus by perceiving the position deviation amount of each transfer-pattern by means of, for example, a so-called overlay inspection system, and applying this result in the linear relation.
According to this method, by inspecting plural transfer-patterns exposed by varying the position of the semiconductor substrate, it skips the procedure of determining the best focal point of the exposure apparatus. That is, the inspection-pattern for measuring the focal point of the exposure apparatus is formed by one exposure, and by measuring this inspection-pattern, the best focal point of the exposure apparatus can be determined.
Similar to the monitoring method for Timothy Brunner et al., recently, a monitoring technique of the focal point of the exposure apparatus by measuring the position deviation amount of patterns by using the overlay inspection system is disclosed by Shuji Nakao et al. of Mitsubishi Electric Corporation in page 733 of Extended Abstracts (The 48th Spring Meeting, 2001); The Japan Society of Applied Physics and Related Societies (March, 2001). In this method, instead of using a special mask having the phase shifter 402b formed therein as in the case above, by using a general mask having an inspection mask-pattern formed therein by an ordinary light permeable film-pattern of chromium, it is intended to monitor the focal point of the exposure apparatus.
In this method, when standardized optically by using coherency σ of an illuminating light source of the exposure apparatus, it is characterized by using an illuminating aperture 501 that can be expressed schematically in the size and shape as shown in FIG. 33. First, the illuminating aperture 501 is disposed at the secondary light source side of the exposure apparatus so that a center of the illuminating light source of the exposure apparatus (not shown) may come to an off-axis point, substantially located off an optical axis of the exposure apparatus. In such off-axis illuminating condition, a pattern of a relatively large size, for example, 2 μm is exposed. Similarly, a pattern of 2 μm is exposed in the illuminating condition in which the center of the illuminating light source may substantially come to the central position of the optical axis. However, when exposing in these two different illuminating conditions, double exposure is executed so that each exposed pattern may be a so-called box-in-box-inspection-pattern. More specifically, double exposure is executed so that the pattern formed in the off-axis illuminating condition may come to the inside box, and that the pattern formed in the axis-center illuminating condition may come to the outside box.
The pattern exposed in the off-axis illuminating condition is deviated in position while keeping the substantially linear relation depending on the deviation amount of the focal point, whereas the pattern exposed in the axis-center illuminating condition is not deviated in position even if the focal point is changed. In this method, therefore, by measuring the relative position deviation of an inside pattern and an outside pattern of the box-in-box-inspection-pattern by a overlay inspection system, it is designed to measure the focal point of the exposure apparatus at the time of exposure.
The reason why this method can be executed is that, when projecting a relatively thick pattern, it is possible to project by the diffraction light near the principal ray only because the ray for illuminating a thick pattern on the mask is hardly diffracted by spreading at a wide angle when passing through the mask. In this method, the pattern formed on the mask may be a pattern made of an ordinary shielding film, and any special phase-shift-pattern is not needed.
In the monitoring method proposed by Timothy Brunner et al., the original plate mask 401 requires a phase shifter 402b for inducing a phase shift of 90 degrees that is not required usually. As a result, the manufacturing cost of the mask is increased.
In the monitoring method proposed by Shuji Nakao et al. of Mitsubishi Electric Corporation, the inspection-pattern (measurement-pattern) cannot be transferred unless double exposures are executed. Therefore, when the focus monitor by this method is applied in the field of mass production, the time required for exposure increases, and the productivity is lowered. To measure the focal point at high precision in this method, it is required to read the position deviation amount of the measurement-pattern at a precision of several nanometers. Accordingly, at the time of double exposure, the mask and transfer substrate must be fixed so as not to be moved between the first exposure and the second exposure. In such a case of reading at a precision of several nanometers, in order to assure the precision necessary for measurement, it is required to continue to hold the position of the mask and transfer substrate at a positional precision of several times higher, that is, 1 nm or less. It is, however, very difficult to continue to hold the position of the mask and transfer substrate (image-receiving element) at such precision even by the latest high control technology.
Further, if these problems exist, it is hard to transfer the mask-pattern in an appropriate shape, or it is difficult to manufacture favorable semiconductor devices capable of exhibiting the desired performance.